GB2447424A

GB2447424A - Drum brake wear adjustor having means to prevent adjustment at high temperature

Info

Publication number: GB2447424A
Application number: GB0705002A
Authority: GB
Inventors: Giovanni Fra
Original assignee: Automotive Products Italia SV SRL
Current assignee: Automotive Products Italia SV SRL
Priority date: 2007-03-15
Filing date: 2007-03-15
Publication date: 2008-09-17
Anticipated expiration: 2027-03-15
Also published as: GB0705002D0; GB2447424B

Abstract

A wear adjustor comprises a first end element 25, an intermediate element 28 which engages the first end element via a screw threaded connection 25a, 28a, and a second end element 27, the second end element slidably connected 27a, 28b, to the intermediate element, a toothed ratchet wheel 30 secured for rotation with the intermediate element, a leaf spring 32 acting between the second end element 27 and the intermediate element 28 and carrying a pawl 31 for engaging the teeth 30a on the ratchet wheel, there existing a heat sensitive bimetallic disabling means 40 which contacts the pawl 31 and forces it out of engagement with the ratchet wheel 30. Preferably, the bimetallic disabling means partially surrounds the intermediate element, and has one end 45 for disengaging the pawl and another end for locking the ratchet wheel to prevent rotation thereof at high temperatures. The bimetal may be mounted on a sleeve 35. The spring may engage the sleeve via a tang 32b.

Description

BRAKE ADJUSTER

This invention relates to an automatic adjuster for the working clearance of the shoes of a vehicle drum brake The European patent EP-B-O 538 909 describes an automatic adjuster of the type, hereinafter referred to as being of the type described, having three coaxial elements comprising a first end element with a head portion for fixing against rotation to a first brake shoe, an intermediate element one end of which engages the first element via a screw threaded connection, a second end element with a head portion for fixing non-rotatably to a second brake shoe and slideably connected to the intermediate element so as to be freely rotatable with respect thereto; a toothed ratchet wheel secured for rotation with the intermediate element, a leaf spring acting between the second end element and the intermediate element to urge the second end element away from the intermediate element, the said leaf spring carrying a pawl element for engaging the teeth of the ratchet wheel; and heat-sensitive bi-metal disabling means for disabling the operation of the adjuster It is known that expansion of the brake drum can cause such an adjuster mechanism to overcompensate when the brake is hot. If this is permitted to occur, all working clearance between the brake shoes and the drum could be lost, meaning that, when the brake cools down, the shoes will still contact the drum thus braking the wheels. Clearance must, therefore be adjusted at relatively low temperatures and adjustment must be prevented at high temperatures.

European patent EP-B-O 538 909 suggests providing the adjuster with heat-sensitive means operable to prohibit adjustment of the shoes clearance at high temperatures which includes a bi-metal strip with one end fixed to the intermediate element of the adjuster and the other end of the bi-metal strip having a precision-shaped head engageable in a locking seat formed in an insert securely fixed to one of the first or second end elements when the temperature rises above a predetermined threshold. In these conditions, the adjuster behaves like a rigid unit which is prevented from lengthening in order to stop the ratchet wheel and pawl from moving and adjusting the effective length of the adjuster.

Whilst this arrangement operates satisfactorily, it requires the manufacture and fitting of small parts which must be of a very precise shape and dimension in order to operate correctly at high temperatures In particular, the head of the bi-metal strip and the seat thereof, formed in an insert fitted into one of the aforesaid end elements, demand very high manufacturing and assembly tolerances, which can only be achieved in complex and costly operations.

It is an object of the present invention to provide an improved automatic adjuster which is more economical to manufacture Thus according to the present invention there is provided a brake adjuster of the type described in which the bi-metal disabling means contacts the pawl and forces the pawi out of engagement with the ratchet wheel to disable the operation of the adjuster at high temperatures Preferably the bi-metal disabling means is of part-annular strip form and partially surrounds the intermediate element, one end portion of the part-annular bi-metal strip being supported from the intermediate element and the other end portion of the strip being arranged to bend away from the intermediate element at high temperatures to contact the pawl and force the pawl out of engagement with the ratchet wheel.

The end portion of the bi-metal strip which is supported from the intermediate element preferably has a locking portion which moves into engagement with the ratchet wheel teeth at high temperatures to prevent relative movement in the screw-threaded connection between the first end element and the intermediate element when the pawl is disengaged from the ratchet wheel teeth This prevents loss of adjustment of the effective length of the adjuster.

The bi-metal disabling means is preferably supported from the intermediate element on a sleeve-like member which surrounds the intermediate member The sleeve-like member may be a push-fit on the intermediate element and has locating formations which snap into a groove formed in the intermediate member to locate the sleeve-like member on the intermediate element Conveniently the part annular bi-metal strip is supported from the intermediate element between the intermediate element and axially extending lugs formed on one end of the sleeve-like member In a preferred arrangement the end portion of the bi-metal strip supported from the intermediate element extends radially between two closely adjacent axially extending lugs on the sleeve-like member and is than bent backwards towards the ratchet wheel teeth to form the locking portion of the bi-metal strip.

The invention also provides an adjuster of the type described in which one end of the leaf spring is connected with the second end element or the intermediate element by providing an aperture in the end of the leaf spring which engages over a projection on the relevant element, the leaf spring including an adjacent tang which, after insertion of the projection through the aperture, is deformed to engage the projection and thus retain the projection in the aperture.

The present invention will now be described, by way of example only, with reference to the accompanying drawings in which:-Figure 1 is a plan view of a vehicle drum brake embodying a brake adjuster in accordance with the invention; Figure 2 is a side view of the adjuster used in the brake of Figurel; Figure 3 is a side view in the direction of the arrow A of Figure 2, Figure 4 is a side view in the direction of the arrow B of Figure 2; Figures 5 and 6 show enlarged sections on the line C-C of Figure 2 with the bi-metal disabling means non-operative and operative respectively, Figure 7 is an enlarged section on the line IJ-D of Figure 2, and Figures 8 and 9 show details of the attachment of one end of the leaf spring onto the adjuster.

Referring to figure 1, this shows a drum brake 10 having a pair of brake shoes 11,12 which at their lower ends react against a fixed abutment 13 and at their upper ends are forced apart by an hydraulic wheel cylinder 14. The shoes are kept in contact with the abutment 13 by a lower return spring 15 and in contact with the wheel cylinder 14 by an upper return spring 16.

Thus during normal service operation the shoes are brought into contact with the inner surface of a brake drum (not shown) in the conventional manner by actuating wheel cylinder 14 whose pistons (not shown) respectively outwardly displace the contacting ends of shoes 11 and 12 to engage the drum. In the conventional manner the two shoes are held down onto a back plate 17 by hold down clips 21 and 22 respectively The drum brake is also provided with a handbrake operating lever 23 pivoted at 24 on brake shoe 12. The hand brake operating lever contacts the right hand end element 25 of an adjuster strut 26 whose left hand end element 27 contacts the other brake shoe 11 In conventional manner if, for example, the handbrake lever 23 is pivoted on pivot 24 in a clockwise sense as viewed in figure 1 (see arrow X) by a cable (shown by dotted detail 23a) the handbrake lever will push the end element of adjuster strut 26 to the left thus pushing the brake shoe 11 to the left. This in effect causes the two brake shoes 11 and 12 to move apart relative to each other and hence engage the inner surface of the brake drum thus applying the hand brake The adjuster strut 26 basically comprises three coaxial components, the right hand end element 25, the left hand end element 27 and an intermediate element 28. The right hand end element 25 has a threaded stem portion 25a which is received in a screw threaded bore 28a in the right hand end of intermediate element 28. Similarly the left hand head element 27 has a stem portion 27a which is axially slideable within an internal bore 28b in the left hand end of intermediate element 28. A ratchet wheel 30 with teeth 30a is formed on intermediate element 28 and is engaged by a pawl 31 which forms part of a bow spring 32. Bow spring 32 is supported at one end 32a on a projection 33 formed on element 27 at its other end 32b on an abutment 34 provided on a sleeve 35 which surrounds the intermediate element 28. Sleeve 35 is slotted at 35a and has depressions 35b which snap into a groove 28c in the intermediate element 28 to locate the sleeve axially on element The end 32a of the bow spring is provided with an aperture 32c (see figure 8) through which the projection 33 extends A tang 32b formed at one end of aperture 32c is bent out of bow spring 32 into contact with projection 33 as shown at 32b' to retain the bow spring on the projection 33 in an undercut portion 33a Bow spring 32 biases left hand end element 27 to the left relative to the intermediate element 28. The effect of this is that when the left hand element 27 is moved into the bore 28b in the intermediate member when the brake shoes are retracted by the springs 15 and 16 this causes the bow spring 32 to assume a more bowed configuration so that the pawl 31 tends to be drawn up out of engagement with the ratchet wheel tooth 30a of ratchet wheel 30 which itis currently engaging When the amount of movement of the left hand head portion relative to the right hand portion exceeds a predetermined level (indicating a predetermined amount of retraction of the brake shoes which in turn indicates a predetermined amount of brake shoe wear) then the pawl will engage the next tooth and ratchet wheel 30 When the brake is next applied the energy stored in the bowed spring rotates the intermediate element 28 slightly relative to the head element 25 to increase the effective length of the adjuster strut 26 to take up the shoe wear.

In accordance with the present invention a temperature sensitive mechanism is provided to ensure disengagement of the pawl 31 from the ratchet wheel 30 at high temperatures to prevent over adjustment of the drum brake.

This temperature sensitive mechanism takes the form of a part annular bi-metal strip which partially surrounds the intermediate element 28 (see Figures 5 and 6). The strip 40 is supported between the element 28 and axially extending lugs 41 formed on the end of sleeve 35. One end portion 42 of the strip 40 extends radially outwardly between two closely adjacent lugs 41a and is then bent backwards at 43 to form a locking portion 44 which engages the teeth 30a of the ratchet wheel at high temperatures to prevent rotation between end element 25 and intermediate element 28 The other end portion 45 of the strip 40 is arranged to bend away form the intermediate element 28 at higher temperatures to the position shown at 45' in Figure 6 in which it contacts the pawl 31 and forces the pawl out of engagement with the ratchet wheel teeth 30a to disable the adjuster.

Thus at higher temperature (that is above 80 C) the pawl 31 is disengaged from ratchet wheel teeth 30a to disable the actuator and the locking portion 44 engages the ratchet teeth 30a to prevent any change in the effective length of the adjuster strut 26.

The present invention thus provides an efficient and cost effective construction for disabling the adjustment of the shoe clearances at high temperatures

Claims

A brake adjuster of the type described in which the bi-metal disabling means contacts the pawl and forces the pawl out of engagement w:th the ratchet wheel to disable operation of the adjuster at high temperatures
2. An adjuster according to claim 1 in which the bi-metal disabling means is of part-annular strip form and partially surrounds the intermediate element, one end portion of the part-annular bi-metal strip being supported from the intermediate element end the other end portion of the strip being arranged to bend away from the intermediate element at higher temperatures to contact the pawl and force the pawl out of engagement with the ratchet wheel.
3. An adjuster according to claim 2 in which the end portion of the bi-metal stip which is supported from the intermediate element has a locking portion which moves into engagement with the ratchet wheel teeth at high temperatures to prevent relative movement in the screw-threaded connection between the first end element and the intermediate element when the pawl is disengaged from the ratchet wheel teeth.
4 An adjuster according to any one of claims 1 to 3 in which the bi-metal disabling means is supported from the intermediate element on a sleeve-like member which surrounds the intermediate member.
5. An adjuster according to claim 4 in which the sleeve-like member is a push-fit on the intermediate element and has locating formations which snap into a groove formed in the intermediate member to locate the sleeve-like member on the intermediate element
6. An adjuster according to claim 5 in which the part annular bi-metal strip is supported from the intermediate element between the intermediate element and axially extending lugs formed on one end of the sleeve-like member
7. An adjuster according to claims 3 and 6 in which the end portion of the be-metal strip supported from the intermediate element extends radially between two closely adjacent axially extending lugs on the sleeve-like member and is then bent backwards towards the ratchet wheel teeth to form the locking portion of the bi-metal strip.
8. An adjuster according to any one of claims 1 to 7 in which one end of the leaf spring is connected with the second end element or the intermediate element by providing an aperture in the end of the leaf spring which engages over a projection on the relevant element, the leaf spring including an adjacent tang which, after insertion of the projection through the aperture, is deformed to engage the projection and thus retain the projection in the aperture.
9. An adjuster of the type described in which one end of the leaf spring is connected with the second end element or the intermediate element by providing an aperture in the end of the leaf spring which engages over a projection on the relevant element, the leaf spring including an adjacent tang which, after insertion of the projection through the aperture, is deformed to engage the projection and thus retain the projection in the aperture
10. A brake adjuster of the type described constructed and arranged substantially as hereinbefore described with reference to and as shown in the accompanying drawings.